1. Field of the Invention
The present invention relates to a fixing device that is disposed in an image forming apparatus such as a copier, a facsimile, and a printer to appropriately heat a recording medium, thereby performing fixing in a fixing nip. The invention further relates to a fixing condition control method and an image forming apparatus having the fixing device.
2. Description of the Related Art
From the past, there has been known a fixing device that is disposed in an image forming apparatus such as a copier, a facsimile, and a printer. In the fixing device, the recording medium such as paper, on which an image formed with the use of toner is carried, passes through a fixing nip that includes a fixing member such as a fixing roller or a fixing belt and a pressing member such as a pressing roller. In the fixing nip, the recording medium is applied with heat and pressure, and as a result, the image is fixed onto the recording medium (for example, see Japanese Patent Application Laid-open No. 2007-183365, Japanese Patent Application Laid-open No. 07-121052, Japanese Patent Application Laid-open No. 07-234606, and Japanese Patent Application Laid-open No. 2007-310337 and Japanese Patent No. 3224169 and Japanese Patent No. 3225156).
In such fixing device, it is very important to appropriately apply, to the recording medium in the fixing nip, a heat quantity necessary to fix the image onto the recording medium. The reason is as follows. For example, if there is a shortage of a heat quantity applied to the recording medium in the fixing nip, the toner is insufficiently melted, so that a fixing failure occurs such as a phenomenon called a cold offset in which the image is not fixed onto the recording medium. On the other hand, if the heat quantity applied to the recording medium in the fixing nip is excessive, a fixing failure occurs such as a phenomenon called a hot offset in which the excessively melted toner is fixed to the fixing member and a phenomenon in which a degree of glossiness deviates from an appropriate range. Further, when the excessive heat quantity is applied, unnecessary power consumption occurs.
Thus, it is required to perceive the heat quantity appropriate for fixing the image onto the recording medium and perform the control thereof, so that the heat quantity provided to the recording medium in the fixing nip may be an appropriate heat quantity.
As a typical technique of controlling an input heat quantity, there has been known a technique for deciding a set temperature of the fixing member necessary for fixing in advance, attaching a temperature detection member such as a thermistor or a thermopile to the fixing member, and controlling power of a heat source so that the temperature of the temperature detection member may be a preset temperature. Further, as a technique of controlling the heat quantity without using the fixing set temperature, there may be considered a technique of controlling the heat quantity provided to the recording medium by changing the speed at which the recording medium passes through the fixing nip.
It has been known that the input heat quantity necessary for the fixing process depends on characteristics of the recording medium such as the basis weight of the recording medium, that is, the weight per unit area, the smoothness, or an amount of moisture content. As the basis weight increases, the heat capacity increases, and thus the input heat quantity needs to be increased. Regarding the smoothness, as the surface of the recording medium becomes rougher, the input heat quantity received in the fixing unit decreases, and thus the input heat quantity needs to be increased. As the amount of moisture content increases, an amount of depriving heat of the recording medium when moisture in the recording medium evaporates increases, and thus the input heat quantity needs to be increased.
It has been known that the basis weight, particularly among the characteristics of the recording medium, is very sensitive to setting of the input heat quantity.
Therefore, the following techniques have conventionally been suggested.
A technique in which the basis weight of the recording medium, on which fixing for image formation is performed, is perceived in advance on the basis of a paper type input by a user, for example, on the basis of whether the recording medium is a thick paper or a regular paper, and then the fixing set temperature is changed according to the perceived basis weight (for example, see Japanese Patent Application Laid-open No. 2007-183365).
A technique in which the basis weight of the recording medium, on which fixing for image formation is performed, is estimated on the basis of a temperature change of the fixing member, and then the fixing set temperature is changed according to the estimated basis weight (for example, see Japanese Patent Application Laid-open No. 07-121052 and Japanese Patent No. 3224169)
Further, a technique of changing the fixing set temperature based on a temperature change of the fixing member has been known (for example, see Japanese Patent Application Laid-open No. 2007-183365 and Japanese Patent Application Laid-open No. 07-234606 and Japanese Patent No. 3225156).
However, in the technique of changing the fixing set temperature using the basis weight of the recording medium perceived based on the paper type input by the user, when the user erroneously recognizes the paper type, there is a problem in that the fixing set temperature becomes inappropriate, so that a fixing failure occurs, and energy is wasted in some cases.
Further, in the technique of changing the fixing set temperature using the paper type of the recording medium estimated based on the temperature change of the fixing member, the temperature of the fixing member may change due to heat transfer of the fixing member itself, for example, heat transfer to the inside of the fixing member as well as absorption of heat by the recording medium. That is, since information that does not relate to the basis weight of the recording medium may be included in information for estimating the basis weight of the recording medium, a degree of accuracy of estimation is low, and the fixing set temperature becomes inappropriate. Thus, there is a problem in that a fixing failure occurs, and energy is wasted. This problem occurs similarly on the other techniques of changing the fixing set temperature based on the temperature change of the fixing member.
As described above, in the conventional technique of perceiving or estimating the basis weight of the recording medium, there has been a problem in recognizing the heat quantity appropriate for fixing. Further, a technique of using the smoothness or the amount of moisture content of the recording medium as information for recognizing the heat amount has not been suggested yet.